Many of the procedures of oilfield well servicing require that fluids and gases mixed with various chemicals and proppants be pumped down the oil or gas well (henceforth called the well) tubing or casing under high Pressures during the operations called acidizing and fracturing. These operations serve to ready the well for Production or enhance the present production of the well.
The components which make up the wellhead such as the valves, tubing hanger, casing hanger, casing head and also the blow out preventer equipment generally supplied by the well servicing company, are usually sized for the characteristics of the well and are not capable of withstanding the fluid pressures at which these operations of fracturing and acidizing are carried out. These wellhead components are available to withstand high pressures, but it is not economical to equip every well with them.
There are many tools which are in use in the field which allow these high pressure fluids and gases to bypass the wellhead components and these tools are generally referred to as wellhead isolation tools or in oilfield terms, tree savers, casing savers and top mounted packers. Some of the most popular in use today would include the authors tools; Mcleod, a Wellhead Isolation Tool, Canadian Patent No. 1217128, U.S. Pat. No. 4657075 this tool being used to isolate the wellhead array from pressure in the casing; McLeod, a Well Casing Packer, Canadian Patent No. 1232536, U.S. Pat. No. 4691770, this tool being used to isolate wellhead equipment from pressure in the casing or tubing, depending on which it is set into; also Bullen, A Well Tree Saver, Canadian Patent No. 194905, this tool being used to isolate the wellhead array from pressure in the tubing; Cummins (Assigned to Halliburton Co.) a Wellhead Isolation Tool and Method of Use Thereof, U.S. Pat. No. 3830304, this tool being used to isolate the wellhead array from pressure in the tubing.
There are other tools operating on the same principle; to insert a mandrel with a sealing nipple on the lower end through the wellhead array and into the tubing or casing below the wellhead, thus isolating the wellhead equipment from the pressure and fluid being pumped into the tubing or casing. The use of these tools in the field is quite common but their ability to seal off only the tubing or the casing (when the tubing has been removed) from the wellhead equipment limits the effectiveness of monitoring the fracturing and acidizing processes and poses problems when stoppages in these processes occur or if the well must be "killed" for some reason. ("Killing" a well is a process whereby weighted fluid is pumped down the well to counterbalance the pressure of the producing formation and stop production. The weighted fluid is usually pumped down the tubing).
It is also desirable from cost and safety standpoints to be able to leave the tubing or as it is sometimes called, the "kill string", in the well during the well servicing.